A great many different flame retardants are known from the prior art and commercially available. Halogen-containing alkyl phosphates, for example tris(chlorethyl) phosphate, tris(chlorisopropyl) phosphate and tris(2,3-dichloroisopropyl) phosphate are often employed as flame retardants in the production of flame-retarded flexible polyurethane foams. These halogen-containing flame retardants are receiving increasing criticism from polyurethane (PUR) processors and end-users and the halogen content in the end product is in some cases limited by third-party certification marks (product labels). The abovementioned flame retardants also lead to increased emission values for the resulting polyurethane foams.
Furthermore, halogen-free flame retardants that may be employed in polyurethane foams include alkyl phosphates (e.g. triethyl phosphate), aryl phosphates (e.g. diphenyl cresyl phosphate) and alkyl phosphonates (e.g. dimethylpropane phosphonate). These compounds are liquid and thus exhibit good processability in the production of the polyurethane foam but due to their relatively low molecular weights they also have a relatively high volatility which has a negative effect on the emission values of the resulting polyurethane foams.
For special applications such as the use of polyurethane foams in automobile interiors for example it is a requirement that emissions of volatile organic compounds (VOC) and/or condensable emissions (fogging) from these polyurethane foams shall not exceed low threshold values.
Fogging refers to the undesired condensation of evaporated volatile constituents from the motor vehicle interior on glass panes, in particular on the windscreen. This phenomenon is quantifiable as per DIN 75 201. The automotive industry typically requires that the fogging condensate as determined by the DIN 75 201 B method may be less than 1 mg.
Also known are the so-called “reactive flame retardants” which bear isocyanate-reactive hydroxyl groups and thus react with the polyisocyanate employed for foam production and are incorporated into the polyurethane. These foams therefore show only small VOC and fogging contributions. These include both halogen-containing and halogen-free phosphorous flame retardants. However, due to their (OH)-functionality said flame retardants in some cases exhibit considerable problems during foam production since they increase the crosslinking density of the PUR matrix. Mechanical properties and/or the processability of the liquid components to afford the finished foam are adversely affected so that the reactive flame retardants can be used in the formulation only to a limited extent.
Also prior art are solid, usually halogen-free flame retardants such as melamine, aluminum or magnesium oxide or ammonium polyphosphates. Solid flame retardants necessitate special metering devices, have a deleterious effect on foam properties and considerably increase the viscosity of the reaction mixture which may in turn result in undesired air inclusions and flow lines in the foam.
EP-A 0170206 discloses a process for producing flame-retarded polyurethane foams. Triaryl phosphate esters are disclosed as suitable flame retardants. The resulting polyurethane foams show a low level of skin staining.
EP-A 1785439 discloses 2-hydroxyalkanephosphonates and/or 3-hydroxyalkanephosphonates as halogen-free flame retardants in polyurethane foams. No information about the mechanical properties of the resulting polyurethanes is provided. However, it is pointed out that the reactive monofunctional flame retardants too are afflicted by the fundamental disadvantages of the reactive flame retardants, albeit in attenuated form. It is known to one skilled in the art that monofunctional additives bring about chain termination in the PUR network and therefore (likewise) compromise the mechanical properties of the resulting foam.
US-A 2014/0179811 discloses the use of mixed-C1-C5-alkylated triaryl phosphates having not more than two unsubstituted phenyl radicals and comprising less than 1 wt % of triphenylphosphine based on the total amount of employed alkylated triaryl phosphates for producing flame-retarded polyurethane or polyisocyanurate foams. US-A 2014/0179811 does not disclose any teaching regarding the volatility of the described flame retardants or the polyurethanes produced therewith. On the contrary, the aim is to avoid triphenyl phosphate which in the specification is said to have deleterious environmental properties.
The constantly increasing requirements of low-emissions polyurethane foams, in particular of flexible polyurethane foams, in special applications such as in automobile interiors for example make it necessary to use special halogen-free flame retardants in the production of polyurethane foams.